1. Field of the Invention
The present invention relates to an image forming apparatus, such a copier, printer, facsimile machine, and multifunctional machines combining the functions of these apparatuses, and an image forming method.
2. Description of the Related Art
Conventionally, various image forming methods employing electrophotography are known, in which the surface of the latent image carrier is charged and the charged surface of the latent image carrier is exposed to form an electrostatic latent image. Then, the electrostatic latent image is developed with toner to form a toner image on the latent image carrier. The toner image is transferred onto a recording media such as paper, etc., either directly or through an intermediate transfer member that acts as an image carrier. The transferred toner image is fixed in place on the medium by heat and pressure by a fixing device, whereby an image is formed on the recording media. Any toner then remaining on the latent image carrier and/or the image carrier after the toner image transfer is cleaned by known cleaning means, for example, blades, brushes, rollers, etc.
If there are irregularities on the recording media on which the image is formed, the protruding portions come into contact with the toner on the intermediate transfer member or on the latent image carrier during the toner transfer process. However, in the recess portion, gaps are formed between the toner on the intermediate transfer member or the latent image carrier and the bottom of the recess of the recording media. The gaps reduce a transfer electric field acting on the toner, and accordingly, the transfer electric field in the recess portions are reduced compared to that in the protruding portion, resulting in unevenness of the transferred image. As the degree of roughness of the recording media increases, the transfer electric field in the recessed portions are reduced significantly, making it difficult to transfer the toner at the recess portion and resulting in streaks in the finished image where no toner image is adhered to the medium.
Furthermore, when the toner has remained in the image forming apparatus for a long time without being consumed for forming a toner image, the toner deteriorates: for example, the toner chargeability changes, or the fluidity is degraded because the external additives attached to the surface of the toner are buried or separated. In the normal transfer process using a DC voltage, the transfer performance of transferring the toner onto uneven recording material is unsatisfactory even with toner that has not deteriorated, however, transfer performance is significantly lowered when the toner is deteriorated.
Therefore, there is a unsolved need for an image forming apparatus that can achieve sufficient image density both at the recessed portions and the protruding portions of the surface of the recording material while reducing the occurrence of white spots, and improving transfer performance to the recording media having unevenness even if the toner is deteriorated, thereby obtaining high quality images without unevenness and white spots or streaks.